An avalanche in motion. (Photograph courtesy of Richard
Armstrong, National Snow and Ice Data Center.)

Mountains attract climbers, skiers and tourists who scramble up and down the
slopes, hoping to conquer peaks, each in their own way. Yet, to do this they
must enter the timeless haunt of avalanches.

For centuries, mountain dwellers and travelers have had to reckon with the
deadly forces of snowy torrents descending with lightning speed down
mountainsides. Researchers and experts are making progress in detection,
prevention and safety measures, but avalanches still take their deadly toll
throughout the world.

Each year, avalanches claim more than 150 lives worldwide, a number that has
been increasing over the past few decades. Thousands more are caught in
avalanches, partly buried or injured. Everyone from snowmobilers to skiers to
highway motorists are caught in the "White Death." Most are fortunate enough to
survive.

This is meant to be a brief guide about the basics of avalanche awareness and
safety. For more in depth information, several sources are listed under "More
avalanche resources" in the last section of these pages. They are all
well-written, highly recommended publications by knowledgeable avalanche and
backcountry experts.

For avalanche classes or instruction, contact a local outdoor equipment store
or ski area.

What is the profile of a typical United States avalanche victim? According to
the Colorado Avalanche Information Center, 89 percent of victims are men, most
victims are between the ages of 20-29 (although the average victim age is 31),
and three-quarters of victims are experienced backcountry recreationists (who
are more likely to enter risky situations).

Climbers, backcountry skiers, and snowmobilers are by far the most likely to
be involved in avalanches. For a breakdown of activities in relation to
avalanche deaths, see the national statistics compiled by the Colorado Avalanche Information
Center.

One of the major reasons for increasing avalanche fatalities is the boom in
mountain industries and recreation. Skiing, hiking and other winter sports draw
millions of people to the mountains. To support these activities, more roads,
buildings, and towns are forced into avalanche prone areas.

Backcountry recreationists are most likely to trigger avalanches as they
cross hazardous terrain. Non-recreational deaths (such as highway motorists or
mountain residents) are often caused when a naturally released avalanche buries
buildings or highways.

Although avalanches can occur on any slope given the right conditions, in the
United States certain times of the year and certain locations are naturally more
dangerous than others. Wintertime, particularly from December to April, is when
most avalanches will "run" (slide down a slope). However, avalanche fatalities
have been recorded for every month of the year.

The highest number of fatalities occurs in January, February and March, when
the snowfall amounts are highest in most mountain areas. A significant number of
deaths occur in May and June, demonstrating the hidden danger behind spring
snows and the melting season that catches many recreationists off-guard. During
the summer months, it is often climbers who are caught in avalanches.

In the United States, 514 avalanche fatalities have been reported in 15
states from 1950 to 1997. Colorado has the infamous reputation for being home to
about one third of those deaths. Western states account for the majority of
fatalities. Northeastern states experience relatively few avalanches in
comparison. Arizona's single avalanche death was an out-of-bounds snowboarder
killed in 1995. For more information, see the statistics compiled by the Colorado
Avalanche Information Center.

While expertise is not a guarantee that you won't be caught in an avalanche,
it does provide some basic knowledge about how to avoid avalanche areas, what
types of weather and terrain signs to watch for, and what to do if you are
caught in an avalanche - all information that may save you or other members of
your party.

The three parts of an avalanche path: starting zone,
track, and runout zone. (Photograph courtesy of Betsy
Armstrong.)

All that is necessary for an avalanche is a mass of snow and a slope for it
to slide down. For example, have you ever noticed the snowpack on a car
windshield after a snowfall? While the temperature is cold, the snow sticks to
the surface and doesn't slide off. After temperatures warm up a little, however,
the snow will "sluff," or slide, down the front of the windshield, often in
small slabs. This is an avalanche on a miniature scale.

Of course, mountain avalanches are much larger and the conditions that cause
them are more complex. A large avalanche in North America might release 300,000
cubic yards of snow. That's the equivalent of 20 football fields filled 10 feet
deep with snow. However, such large avalanches are often naturally released.
Skiers and recreationists are usually caught in smaller, but often more deadly
avalanches.

Slab avalanches are the most common and most deadly avalanches, where layers
of a snowpack fail and slide down the slope. Since 1950, 235 people in the U.S.
have been killed in slab avalanches. Hard slab avalanches involve large blocks
of snow and debris sliding down a slope. In soft slab avalanches, the snow
breaks up in smaller blocks as it falls.

An avalanche has three main parts. The starting zone is the most
volatile area of a slope, where unstable snow can fracture from the surrounding
snowcover and begin to slide. Typical starting zones are higher up on slopes,
including the areas beneath cornices and "bowls" on mountainsides. However,
given the right conditions, snow can fracture at any point on the slope.

The avalanche track is the path or channel that an avalanche follows
as it goes downhill. When crossing terrain, be aware of any slopes that look
like avalanche "chutes." Large vertical swaths of trees missing from a slope or
chute-like clearings are often signs that large avalanches run frequently there,
creating their own tracks. There may also be a large pile-up of snow and debris
at the bottom of the slope, indicating that avalanches have run.

The runout zone is where the snow and debris finally come to a stop.
Similarly, this is also the location of the deposition zone, where the snow and
debris pile the highest. Although underlying terrain variations, such as gullies
or small boulders, can create conditions that will bury a person further up the
slope during an avalanche, the deposition zone is where a victim will most
likely be buried.

Several factors may affect the likelihood of an avalanche, including weather,
temperature, slope steepness, slope orientation (whether the slope is facing
north or south), wind direction, terrain, vegetation, and general snowpack
conditions. Different combinations of these factors can create low, moderate or
extreme avalanche conditions.

Keep in mind that some of these conditions, such as temperature and snowpack,
can change on a daily or even hourly basis. This necessitates constant vigilance
of your immediate surroundings while doing any wintertime backcountry travel.
The route you chose may be safe when you begin, but may become dangerous if
conditions change dramatically throughout the day.

While this may seem like a lot of work, once you understand factors that can
cause avalanches, most of these signals require simple observation to evaluate
your surroundings as they change. Simply ask yourself, when are conditions
sufficient to cause a mass of snow to slide down a slope?

The following factors often occur in combination to produce an avalanche, but
if a slope is unstable in any way, it may take only the weight of one skier to
set off an avalanche. The more foresight you have about conditions and
situations to avoid the safer your outing will be.

Weather

Avalanches are most likely to run either during or immediately after a storm
where there has been significant snowfall. The 24 hours following a heavy
snowstorm are the most critical. Consequently, it becomes important to be aware
of current weather conditions as well as the conditions from the previous couple
of days. Temperature, wind, and snowfall amount during storms can create fatal
avalanche conditions during your outing. If there has been heavy snowfall the
day or night before your trip, it may be wise to postpone the trip in order to
avoid the increased avalanche danger.

Snowfall

Recent snowfall puts extra stress on the existing snowpack, especially if it
does not adequately bond to the pre-existing surface layer. The extra weight of
new snow alone can cause a slab to break off and fall down the slope,
particularly in storm-induced avalanches. Snowfall amounts of one foot or more
(frequent in mountainous areas) create the most hazardous situations, producing
avalanches that are often large enough to block highways and cause major
destruction. Amounts of six to twelve inches pose some threat, particularly to
skiers and recreationists. Amounts less than six inches seldom produce
avalanches.

Temperature

Because snow is a good insulator, small temperature changes do not have as
much effect on snowpack as larger or longer changes do. For instance, shadows
from the sun crossing the snow surface throughout the day will not significantly
change snowpack stability. Changes that last several hours or days, such as a
warm front moving through, can gradually increase temperatures that cause
melting within the snowpack. This can seriously weaken some of the upper layers
of snow, creating increased avalanche potential, particularly in combination
with other factors.

When temperatures rise above freezing during the daytime and drop back down
again at night, melting and re-freezing occurs, which can stabilize the
snowpack. This is particularly common during the springtime. When temperatures
stay below freezing, especially below zero degrees Fahrenheit, the snowpack may
remain relatively unstable.

Wind scouring snow off of the windward side of the peak
and depositing it on the leeward side. (Photograph courtesy of Richard
Armstrong.)

Wind direction

Wind usually blows up one side of a slope or mountain (the windward side),
and down the other (the leeward side). Blowing up the windward slope, wind will
"scour" snow off the surface, carry it over the summit, and deposit it on the
leeward side. What this does is pack snow unevenly on the leeward side, making
it more prone to avalanche. A cornice or icy overhang at the top of a mountain
or ridge is a telltale sign of wind scouring. It is safer to travel on the back,
or windward side of such a slope, where the snow layer is thinner and
wind-packed.

Although it seems like a small amount because the snow may look light and
powdery, the weight can add up significantly and can be a critical factor if a
slope is already unstable. In the Northern Hemisphere, storms generally move
from west to east. Consequently, the leeward slopes are most often the
northeast, east, and southeast facing slopes. These slopes become easily
wind-loaded and will more readily avalanche. Many ski areas are built on slopes
with these orientations and must use prevention measures to counteract the
natural avalanche conditions that build up on these slopes.

Snowpack conditions

Perhaps the most significant factor (but not the only one) is how the
snowpack has developed over the season. We only see the surface and maybe the
top few layers of snow, but it can be layers of snow several feet deep that may
ultimately determine whether the slope will fail.

A new layer of surface hoar on the snow. Note the
quarter for scale. (Photograph courtesy of K.
Williams.)

Understanding the history of snowpack for that season can reveal several
clues about slope stability. The snowpack as a whole may change not only during
the course of the winter season, but throughout the course of a single day, due
to changing weather and temperature conditions. This is why constant awareness
and frequent slope testing are necessary.

Snowpack conditions are extremely important because many layers of snow build
up over the winter season. Each layer is built up under different weather
conditions and will bond differently to the subsequent layers. Snowflakes, or
snow crystals, within the snowpack eventually become more rounded due to
melting/re-freezing and settlement. This metamorphism allows them to compress
and (generally) form stronger bonds.

In between snows, the temperature may rise and melt the exposed surface
layers, which when they re-freeze create a smoother, less stable surface for the
next snowfall. Failure is much more likely to occur during or after the next few
snowfalls. Rain between snows creates a slicker surface as well, and can weaken
the bonds between snow layers. On the other hand, light snowfalls and
consistently cold temperatures help strengthen the snowpack and make it more
resistant to avalanche. Weak layers deep in the snowpack can cause avalanches
even if the surface layers are strong or well bonded.

A type of snow called depth hoar (a course, grainy form of snow
crystal) is often the culprit behind avalanches. Because of its granular
structure, similar to dry sand, depth hoar bonds poorly and creates a very weak
layer in the snowpack. Unfortunately, the weather conditions necessary to
produce depth hoar most often occur very early in the season, and these weak
layers are buried under subsequent snows. All too often, deeper depth hoar
layers are discovered only after an avalanche has swept off the overlying
layers.

Slope angle

Most avalanches occur on slopes between 30 and 45 degrees, but can occur on
any slope angles given the right conditions. Very wet snow will be well
lubricated with water, meaning it might avalanche on a slope of only 10 to 25
degrees. Very dry or granular snow will most likely avalanche on a slope close
to the 22 degree angle of repose. Compacted, well-bonded layers create a
snowpack that can cling to steeper slopes until a weak layer is created.

You can measure the slope angle with an inclinometer, or you can "eyeball" it
by dangling a ski pole by the strap and estimating the angle. Of course, you may
want to practice before using this technique in the backcountry to be sure of
your accuracy. Be aware that a single slope can have varying degrees of
steepness across its face, depending on the terrain. You may start out on a
gentle 25 degree angle, but as you cross, the slope may steepen significantly
enough to become an avalanche hazard.

Slope orientation

Although avalanches will run on slopes facing any direction, most avalanches
run on slopes facing north, east, and northeast (also the slope directions that
most ski areas are located on). Because the sun is at such a low angle,
particularly during the winter, a colder and deeper snowpack develops. Slopes
that are under shadow throughout most of the day are suspect because the
snowpack remains cooler, without much of the melting and bonding that can make
the snow layers stronger.

Remember also that certain slope orientations are much more affected by
wind-loading, particularly northeast, east, and southeast (similar to the
orientations mentioned above). If you are not already familiar with the terrain,
taking a compass along would be a good idea. Alternatively, if you know where
you are going ahead of time, you could potentially plan your route in such a way
as to avoid suspect slope orientations, especially if other potential avalanche
factors exist.

Terrain

Paying attention to where you are in the grand scheme of things can offer
clues about avalanche likelihood. Bowls and gullies are suspect at any time,
regardless of other conditions. Snow can accumulate deeply and quickly in these
areas, increasing the possibility of an avalanche. Even if you can see that an
avalanche has already run, be wary. Avalanches can fall in a "piecemeal"
fashion, where one avalanche will run and leave the rest of the slope weakened,
and the slightest provocation can cause subsequent avalanches on that same
slope. Smaller depressions or shallow gullies in the mountainside can also be
hazardous. During an avalanche, these "terrain traps" serve as accumulation
points for snow and debris in which a victim could be buried.

Crossing steep slopes where you may trigger the avalanche yourself should be
done cautiously. In contrast, as you cross a valley floor you may also be caught
in an avalanche triggered naturally on the steep slope above you. Therefore,
during hazardous conditions minimize the amount of time traveling beneath
avalanche starting zones and never camp in a potential avalanche runout zone.
Even a small avalanche starting high on the slope can carry down large amounts
of snow onto and across the valley floor. Remember to keep an eye out for
obvious avalanche chutes, where avalanches occur more frequently.

Vegetation

On a snow-covered slope, heavily forested areas are much safer than open
spaces, but don't assume that any vegetation at all will be protective. Lone
trees, bushes, or large rocks on a mountainside can sometimes weaken the
stability of the snowpack. A fracture line (the break-off point for an
avalanche) may run from a lone tree to a rock to another tree. Also, during
avalanches, trees and rocks catch debris and cause excessive snow pile-up, as
well as provide lethal obstacles for anyone caught in an avalanche.

Tree line, above which conditions become too harsh for trees to grow, also
plays a significant role in avalanche areas. Many avalanches start above the
tree line, making high-elevation mountains especially risky. Although forests
help stabilize the snowpack, if an avalanche starts above tree line, it can cut
its own path, or chute, through the trees below. Likewise, where there is a
swath of trees missing from a forested mountainside (and it's not a ski run),
there are probably frequent avalanches running down that particular chute.

Smooth surfaces, such as a rock face or grassy slope, may cause avalanches
during the spring melting season. On the other hand, if the vegetation is very
low-lying, such as tree stumps or shrubs, it can become buried underneath the
first few snows and be relatively ineffective at anchoring the upper layers of
the snowpack.

There are several ways to gauge snowpack stability. Keep any eye out for any
cracks shooting across the surface, or small slabs shearing off. These are signs
of weakened snowpack. Also, listen for "hollow" or "whumping" noises as you walk
or ski. This indicates that there is a weaker layer underneath, leaving the
surface layer more prone to collapse. Careful, continuous observations
throughout your trip can reveal natural clues, but other more reliable
measurements, such as snow pits and shear tests, will help you predict more
accurately how stable or unstable the snowpack is.

Snowpits

Digging a snowpit reveals more about the snowpack structure than is visible
from the surface. Making a snowpit requires a little more practice and
experience, but it is quickly accomplished with a portable shovel. The most
effective snowpits should be dug near potential avalanche starting zones, but
without putting you or other members of your party at risk. With a shovel, dig a
hole four to five feet deep and about three feet wide. Smoothen the uphill wall
until it is vertical and you can see the different layers of snow. By pressing
your hand against each layer to feel its hardness, you can determine whether
there are weak layers.

Shear tests

Once you have dug the pit, a shear test is fairly easy. From the vertical,
uphill wall, separate a column of snow without pulling it free from the wall.
Insert a shovel at the back (uphill side) of the column and gently pull on the
handle. If weak layers pull loose quite easily, the snowpack is very unstable.
If it takes a few tugs on the handle before any layers pull loose, the snowpack
is slightly unstable. If you really have to pry hard on the handle to loosen any
layers, the slope is relatively stable, although caution should still be used at
all times.

When conducting these tests throughout the day, pay attention to the slope
angle. Layers that seem strong on a 30-degree slope may be much weaker on a
steeper slope. Also, remember that the shear test relies on the pull of a
shovel, not the weight of a person. You can test this by standing or jumping on
the uphill edge of the snowpit (the "banzai jump" test), but only if you already
know the snowpack is stable after conducting a shear test and if this presents
no risk of injury or of triggering an avalanche.

Snowpit and shear tests should be conducted frequently during your outing,
especially if you are crossing several different slopes or types of terrain.
Some experts perform them dozens of times a day. While this may seem a bit
time-consuming at first, experience will speed the process. More importantly, it
is a fairly simple and accurate measure you can make which may save your
life.

Ideally, avoiding avalanches in the first place is much easier than trying to
survive one. Avalanche safety begins even before you begin your travel. In
addition to keeping an eye out for weather and terrain conditions, there are
steps you can take ahead of time to help you or other members of your party if
you are caught in an avalanche.

Proper equipment can be a critical factor in rescue efforts. Avalanches kill
in two ways. A victim will either endure fatal trauma (collisions with rocks or
trees) during an avalanche, or will suffocate after they are buried by snow.
While trauma deaths occur before rescue can take place, the more common
suffocation deaths are often tragic because with the proper equipment and
expertise, they can be avoided.

Portable shovels made of plastic and aluminum are lightweight and
compact enough that they can be carried in a pack. Digging with a shovel, as
opposed to using hands or ski poles, can dramatically decrease the time it takes
to dig out a victim. Digging by hand takes an average of 45 minutes to dig out
one square meter of snow. Using a shovel to dig out the same amount of snow
takes less than ten minutes.

Collapsible probes or ski-pole probes are also easy to carry along.
Collapsible probes usually consist of two-foot lengths of tubular steel that
join together to make a probe ten to twelve feet long. Ski-pole probes are made
so that grips and baskets can be removed. The two poles can then be joined
together to form a probe. Probing is essential to finding a buried victim if
there are no visible clues on the surface.

Avalanche beacons (transceivers) are the most commonly used rescue
device, and are standard equipment for ski-area patrollers and heli-ski
operators. When properly used, they provide the fastest way of locating a
victim. When a victim is buried, the transceiver will emit a frequency that
other transceivers can home in on. However, it is critical to have the
transceiver set to "transmit" during your outing. When trying to locate a buried
victim, rescuers will then switch their transceivers to "receive" to locate the
signal. Unfortunately, avalanche deaths have occurred due to the fact that the
victims had their transceiver switched to "receive" rather than "transmit."
Consequently, rescuers could not locate them in time.

Remember that more than one transceiver unit is required. A transceiver will
not help locate a victim who is not also wearing one. Likewise, a victim with a
transmitting beacon may not be found unless someone else has a transceiver to
pick up that signal.

Using beacons requires practice. Homing in on a buried signal involves moving
in increasingly smaller circles around the area of the signal. When purchasing a
unit, learn how to use it properly, and practice using it frequently. Make sure
those in your party carrying transceivers understand how to use them.

Time is of the essence. Carrying this equipment may mean the difference
between life and death for someone buried in an avalanche. Statistics show that
most survivors are dug out within 15 to 30 minutes. For victims buried longer
than 30 minutes, survival chances decrease drastically. In fact, U.S. statistics
show that victims buried longer than 45 minutes rarely survive. Depth of burial
is also a factor in surviving, but even if a victim is near the surface, the
length of time it takes to locate them and dig them out can still be the
critical factor.

Before crossing a slope where there is any possibility of an avalanche,
fasten all your clothing securely to keep out snow. Loosen your pack so that you
can slip out of it with ease and remove your ski pole straps. Make sure that
your avalanche beacon is on and switched to "transmit" rather than "receive."
Cross the slope one at a time to minimize danger.

If you are caught in an avalanche

Yell and let go of ski poles and get out of your pack to make yourself
lighter. Use "swimming" motions, thrusting upward to try to stay near the
surface of the snow. When avalanches come to a stop and debris begins to pile
up, the snow can set as hard as cement. Unless you are on the surface and your
hands are free, it is almost impossible to dig yourself out. If you are
fortunate enough to end up near the surface (or at least know which direction it
is), try to stick out an arm or a leg so that rescuers can find you quickly.

If you are in over your head (not near the surface), try to maintain an air
pocket in front of your face using your hands and arms, punching into the snow.
When an avalanche finally stops, you will have from one to three seconds before
the snow sets. Many avalanche deaths are caused by suffocation, so creating an
air space is one of the most critical things you can do. Also, take a deep
breath to expand your chest and hold it; otherwise, you may not be able to
breathe after the snow sets. To preserve air space, yell or make noise only when
rescuers are near you. Snow is such a good insulator they probably will not hear
you until they are practically on top of you.

Above all, do not panic. Keeping your breathing steady will help preserve
your air space and extend your survival chances. If you remain calm, your body
will be better able to conserve energy.

Rescuing a victim

Try to watch the victim as they are carried down the slope, paying particular
attention to the point you last saw them. After the avalanche appears to have
finished and settled, wait a minute or two and observe the slope carefully to
make sure there is no further avalanche danger. If some danger does still exist,
post one member of your party in a safe location away from the avalanche path to
alert you if another avalanche falls.

When traveling with a large party, you may want to send someone for help
immediately while the rest of you search. If you are the only survivor, do a
quick visual search. If you don't see any visual clues, and you don't have
transceivers, then go for help.

Begin looking for clues on the surface (a hand or foot, piece of clothing,
ski pole, etc.), beginning with the point where they were last seen. As you move
down the slope, kick over any large chunks of snow that may reveal clues. Since
equipment and items of clothing may be pulled away from a victim during an
avalanche, they may not indicate their exact location, but can help determine
the direction the avalanche carried them. Mark these spots as you come across
them. Be sure that all rescuers leave their packs, extra clothing, etc., away
from the search area so as not to clutter or confuse search efforts.

Once the victim is found, it is critical to unbury them as quickly as
possible. Survival chances decrease rapidly depending on how long a victim
remains buried. Treat them for any injuries, shock, or hypothermia if
necessary.

If you lost sight of the victim early during the avalanche, or if there are
no visible clues on the surface, mark where the victim was last seen. Look at
the path of the snow and try to imagine where they might have ended up. For
those wearing avalanche transceivers, switch them to "receive" and try to locate
a signal.

For those using probes, begin at the point the victim was last seen at. Or if
you have a good idea of where they were buried, begin in that area. Stand in a
straight line across the slope, standing shoulder to shoulder. Repeatedly insert
the probes as you move down slope in a line. Pay particular attention to shallow
depressions in the slope and the uphill sides of rocks and trees, since these
are terrain traps where they may have been buried.

It may be necessary to probe certain areas more than once if you don't locate
the victim the first time around, but this takes more time and decreases the
victim's chances for survival. Similar to using transceivers, this method of
rescue is much more effective if those involved have experience or have
practiced finding buried victims using probes.

After searching for clues, or using transceivers and/or probes, still does
not reveal the location of the victim, it may be time to rely on outside help.
Nearby ski resorts will be staffed with personnel experienced to handle these
situations. They will have equipment to locate the victims and dig them out (if
your party did not bring shovels or probes), and they may also have avalanche
dogs that can help find victims. Ski area patrollers will also have first aid
equipment, but unfortunately, by the time they can usually reach out-of-bounds
avalanche accidents, too much time has elapsed to save the victim.